Spatial variations in nutrient supply to the red algae Eucheuma serra (J. Agardh) J. Agardh.

The nutrient supply rates within the canopy of the economically important red algal species, Eucheuma serra J. Agardh were determined experimentally in a recirculating flow-chamber. A single individual was placed in the working section of the 2000 × 200 × 250 mm3 acrylic flow-chamber and subjected to unidirectional water velocities from 1.0 to 9.3 cm s−1. Rates of nutrient supply were determined using 9.7 mm diameter CaSO4 (gypsum) spheres that were attached to the thallus inside and outside of the canopy. The supply rates within the canopy were 56% less than outside of the canopy. Increases in internal and external water velocity asymptotically increased the nutrient supply rates regardless of location. A model was developed to examine how changes in ammonium and nitrate supply compared with the physiologically maximum uptake rates of these nutrients. The results suggest that when the ammonium concentration in the water was 20 µmol L−1 uptake rates were limited by the supply rate especially at velocities below 5 cm s−1, whereas in the case of 20 µmol L−1 of nitrate, the supply of nitrate was more than adequate to maximize nutrient uptake. [ABSTRACT FROM AUTHOR]